The invention relates to a tuner with at least a first and a second frequency band and with a signal input terminal for the supply of a frequency information signal.
The invention further relates to an integrated circuit with a PLL circuit for controlling a tuner which comprises at least a first and a second frequency band.
Such a tuner and such an integrated circuit are known, for example, from the tuner UV 316 of the Philips company. This known tuner is manufactured in accordance with the WSP (World Standard Pinning) standard. The WSP standard lays down the mechanical dimensions, the functions of the individual connection terminals of the tuner, as well as the antenna height. Terrestrial tuners usually comprise two or three frequency bands, each comprising a tunable input filter, a controllable amplifier, a tunable band-pass filter, and a mixer circuit. Three pins are available for the supply of one control signal each for the control and selection of the frequency bands in the WSP. The control of the tuner takes place by means of the microprocessor of the TV set which is coupled to the tuner, for example, by means of a I2C bus. A table is stored in known TV sets for the selection of the frequency band, by means of which table the frequencies of the TV signal which can be selected are associated with the respective frequency band of the tuner. When the user of the TV set selects a TV signal frequency, for example by means of a remote control unit, the microprocessor of the TV set selects the associated frequency band of the tuner from the table and transmits the control signals defining the band selection to the tuner.
The selection of the frequency bands of the tuner by means of the microprocessor of the TV set has the disadvantage that only tuners can be used for the TV set whose frequency bands correspond to the values stored in the table of the TV set. A subsequent change in the tuner as regards the characteristics of the frequency bands, or the use of a different tuner is no longer possible.
It is an object of the invention to provide a tuner as well as an integrated circuit of the kind mentioned in the opening paragraphs which are universally applicable.
According to the invention, this object is achieved with regard to the tuner in that the tuner comprises a band selection circuit which is provided for the selection and control of one frequency band at a time, and in that the tuner comprises a frequency search function for tuning to a frequency, which function in a first step is designed to test the tuning possibility of the frequency in the first frequency band and, if tuning in the first frequency band is not possible, in a second step is designed for testing the tuning possibility of the frequency in the second frequency band.
The tuner according to the invention thus evaluates the frequency information signal supplied to it by the microprocessor of the TV set and independently selects the frequency band in dependence on this frequency information signal. To render it possible for the tuner to select the frequency band independently, this tuner comprises a frequency search function. This frequency search function is designed such that it is consecutively tested in at least two different frequency bands whether a desired frequency can be tuned in the relevant frequency band.
Each frequency band has a mixer circuit with an oscillator for driving the mixer. The oscillator preferably comprises an oscillation circuit with a controllable capacitance. The controllable capacitance is preferably implemented as a reverse-biased varicap diode whose capacitance depends on the bias voltage applied. The oscillator frequency of the oscillation circuit can be controlled through a change in the bias voltage and the resulting change in the capacitance. Terrestrial tuners are usually designed for the conversion of input frequencies in a range from 45 MHz to 863 MHz. Since the controllable or adjustable capacitance range of the varicap diodes is limited, the tuners preferably comprise three frequency bands, each with its own oscillation circuit and with its own varicap diode designed for tuning the oscillation circuit. Testing of the tuning possibility accordingly means that it is tested whether the oscillator of the relevant frequency band can be tuned to the desired frequency which is necessary for mixing to the intermediate frequency of the tuner. The oscillation circuits are preferably coupled to a quartz oscillator by means of a programmable divider circuit. To synchronize the oscillation circuit with the quartz oscillator, a phase-locked loop (PLL) is preferably provided. The test as to whether a desired frequency can be tuned in one of the frequency bands may take place by means of a latching detector which detects whether the phase control circuit latches or locks itself, i.e. whether the desired oscillator frequency of the oscillation circuit can be synchronized with the reference frequency of the quartz oscillator, a programmable divider circuit being provided between the oscillation circuit and the quartz oscillator.
The individual frequency bands usually each comprise a band-pass filter which is formed by a coil and a varicap (variable-capacitance) diode connected in parallel thereto. The barrier layer capacitance of the varicap diode is dependent on the applied bias voltage and can thus be controlled through a change in the applied bias voltage. The varicap diodes of the band-pass filter and the varicap diodes of the oscillation circuits are preferably controlled in parallel during the tuning.
When a frequency information signal with a desired frequency is supplied to the tuner, the oscillation frequency of the oscillator of the first frequency band is scanned from the lowest band frequency to the highest band frequency in a first step. It is tested during this whether the desired frequency is tunable in the first frequency band. If the desired frequency could not be tuned in the first frequency band, a switch-over to the second frequency band takes place automatically, where the respective oscillator again is scanned continuously from the lower band limit to the upper band limit for tuning to the desired frequency.
This principle may also be applied in tuners having three or more frequency bands, for example, in the case of three frequency bands, the tuning is attempted first in the first, then in the second, and finally in the third frequency band.
The independent band selection of the tuner by means of the band selection circuit and the frequency search function has the advantage that a tuner of this construction can be used in any TV set whatsoever. Neither the software of the microprocessor of the TV set nor the software of the tuner need be changed for this purpose. Such a tuner can thus be manufactured efficiently and inexpensively in very large quantities. It is in addition possible to change the physical parameters of the individual frequency bands without a change in the software of the tuner being necessary. For example, the band boundaries of the frequency bands can be shifted through the use of different varicap diodes or other inductances for the respective band-pass filters and oscillation circuits. No change in the tuner software is necessary in such a case, because the tuner changes the switch-over points between the individual frequency bands automatically.
An example embodiment of the invention renders it possible to avoid an unnecessary switching-over between the frequency bands. The band selection circuit determines a tuning mode in accordance with the frequency difference between the old and the new frequency information signal as determined by the frequency difference detection circuit, which signal is supplied to the tuner, for example, by the microprocessor of the TV set. This tuning mode influences the further band selection, i.e. it determines whether and under what conditions a band switch-over between the frequency bands will take place.
According to an example embodiment, tuning modes which can be distinguished by the frequency difference detection circuit are, for example, a refresh mode, and/or an automatic frequency control mode, and/or a search mode, and/or a new-channel selection mode. It is detected in the refresh mode that the microprocessor of the TV set has once more sent the same frequency information signal to the tuner. In such a case, a band switch is not desirable, and accordingly no band switch is provided for in the refresh mode. In the automatic frequency control mode, the frequency difference detection circuit recognizes that the frequency information signal supplied to the tuner differs only very little from the old frequency information signal. This is characteristic of the automatic frequency control mode of a TV set. In such an automatic frequency control mode, the intermediate frequency demodulator sends a control signal for frequency fine tuning, which is supplied to the microprocessor of the TV set. The microprocessor of the TV set then supplies an only very slightly changed frequency information signal to the tuner. Such a case can be detected by the frequency difference detection circuit, and an unnecessary band switch can be avoided. Advantageously, a further tuning mode which can be recognized by the frequency difference detection circuit is a search mode. TV sets comprise such a search mode for systematically scanning through the individual frequency bands in search of TV channels which can be received. In such a case, the microprocessor of the TV set continuously changes the frequency information signal supplied to the tuner in programmable frequency steps. When a receivable channel is detected, the frequency data of this channel can be stored in a memory automatically or manually. When the microprocessor of the TV set carries out such a search mode operation, a frequency band switch is only desired when the end of the present frequency band has been reached. A frequency band switch is accordingly provided only upon reaching of the end of a frequency band in the search mode of the tuner. Furthermore, the frequency difference detection circuit advantageously is capable of distinguishing a new-channel selection mode. In this new-channel selection mode, the band selection circuit recognizes that a frequency information signal corresponding to a new channel is supplied to the tuner. This is recognized from the fact that the frequency difference between the old and the new frequency information signal exceeds a programmable minimum value. The frequency band setting in the new-channel selection mode takes place by means of the frequency search function, which first tests whether the frequency belonging to the desired channel can be tuned in the first frequency band. If this is not possible, a tuning is attempted in the second frequency band in a second step. The band switch between the first and the second frequency band takes place automatically upon reaching of the band end. The frequency difference characteristic of the search mode lies above the frequency difference of the automatic frequency control mode and below the frequency difference of the new-channel selection mode.
According to an example embodiment, the band selection circuit comprises a band preselection circuit for the adjustment of a new channel, i.e. in the new-channel selection mode. This band preselection circuit subdivides the frequency information signals supplied to the tuner into unequivocal and equivocal frequency information signals. In the case of unequivocal frequency information signals, the band preselection circuit makes an unequivocal and final decision on the frequency band to be assigned to the frequency information signal. In the case of equivocal frequency information signals, it is tested in a first step by means of the frequency search function whether the frequency belonging to the desired channel can be tuned in the first frequency band. If the desired channel cannot be tuned in the first frequency band, the tuner switches automatically to the second frequency band and tests whether the desired channel can be tuned in the second frequency band.
In an example embodiment of the band preselection circuit, the band preselection circuit classifies the frequency information signals of the at least two frequency bands into at least two groups. The first group comprises those frequency information signals which can be unequivocally associated with the first frequency band as well as those frequency information signals whose accompanying reception signal could lie both in the first and in the second frequency band. The second group comprises those frequency information signals whose accompanying reception signal must unequivocally lie in the second frequency band. This means that a frequency band switch-over can only be necessary within the first group, if at all. If the band preselection circuit determines that a frequency information signal belongs to the first group, it is first tested whether the frequency can be tuned in the first frequency band. If the desired frequency cannot be tuned in the first frequency band, the tuner switches automatically to the second frequency band and attempts the tuning in the second frequency band. In the second group of frequency information signals, a switch is immediately made to the second frequency band, and no automatic band switch-over is provided.
An alternative subdivision of the frequency information signals into three groups is provided in an example embodiment. Here the first group comprises those frequency information signals which can be unequivocally assigned to the first frequency band, and the third group comprises those frequency information signals which can be unequivocally assigned to the second frequency band. A second group of frequency information signals is provided between the former two, where the relevant frequency signals may lie either in the first or in the second frequency band of the tuner. This means that no automatic band switch-over by means of the frequency search function is available in the first and the third group. In the middle, second group, however, it is first tested in the first frequency band whether a frequency signal belonging to the frequency information signal can be tuned and, if unsuccessful in the first frequency band, an automatic switch is made to the second frequency band.
In an example embodiment, the bias voltage (or tuning voltage) applied to the varicap diodes of the respective band-pass filters and the respective oscillation circuits is evaluated by a band end detector. This band end detector compares the tuning voltage with a programmable threshold value and supplies a control signal when this threshold value is exceeded. The tuner then switches automatically to the next frequency band and tries to tune the desired frequency in the next frequency band.
Switching-over between the individual frequency bands may advantageously be carried out by means of an example embodiment. In this embodiment, the band selection circuit comprises a cyclical shift register with n elements, n corresponding to the number of frequency bands of the tuner. Thus, for example, a shift register with three elements is provided for a tuner with three frequency bands. Each of these three elements contains a control information, preferably of one bit each, which is provided for controlling a respective one of the three frequency bands. Two of the three elements contain, for example, the control information xe2x80x9c0xe2x80x9d, which means that the frequency band belonging to this element should not be switched on. One of the three elements contains the control information xe2x80x9c1xe2x80x9d at any time, which means that the frequency band belonging to this element is switched on and that a tuning to the desired frequency is to be carried out in this frequency band. It is safeguarded thereby that only one of the three frequency bands is switched on at any time and that the tuning process is carried out in only one of the three frequency bands. When the band end detector detects the end of the respective frequency band, it supplies a control signal to the cyclical shift register, with the result that the control data are passed on to the respective adjoining control element. This means that the frequency band switched on until now takes over the control information xe2x80x9c0xe2x80x9d from the previous control element and is switched off. The control information xe2x80x9c1xe2x80x9d is passed on to the next control element, so that the frequency band belonging to the next control element is switched on, and the tuning to the desired frequency is attempted in this frequency band.
Such an organization of the frequency band switching by means of a shift register can be realized in a simple manner and is very reliable.
The first frequency band, which is scanned first during setting of a new channel, preferably lies below the second frequency band according to an example embodiment. This is particularly advantageous in tuners with three frequency bands in which the band switch-over is realized by means of a shift register, because it renders possible a simple shifting of the control information at the band end.
The tuners according to the invention are preferably constructed such that an overlap is provided in the boundary region between two frequency bands, so that in this overlap region a frequency can be tuned both in the first frequency band and in the second frequency band. In the overlap region, the noise is lower in the lower frequency band than in the higher frequency band, and the gain is higher in the lower frequency band than in the higher frequency band. It is accordingly advantageous in the case of equivocal frequency information signals to start the tuning process in the lower frequency band and, if no result is obtained in the lower frequency band, to switch over to the next higher frequency band.
The tuners according to the invention are preferably utilized in TV appliances.
The object of the invention as regards the integrated circuit according to the invention is achieved in that the integrated circuit comprises a band selection circuit which is designed for supplying control signals for the selection of a frequency band in dependence on the frequency information signal, and in that said band selection circuit comprises a frequency search function for tuning to a frequency, which function in a first step is designed for testing the tuning possibility of the frequency in the first frequency band and, if tuning in the first frequency band is not possible, in a second step is designed for testing the tuning possibility of the frequency in the second frequency band.
Such an integrated circuit may be used for tuners with various frequency band subdivisions, without any change in the software being required. Such an integrated circuit may accordingly be manufactured in very large quantities.